Homopolymer polystyrene resin is typically a rather brittle resin having a poor impact strength. It has long been known that impact strength of polystyrene can be greatly improved by the addition of rubbery particles dispersed throughout the polystyrene resin. Polystyrene resins of improved strength achieved by addition of rubbery particles is often referred to as high impact strength polystyrene (HIPS). The size of the rubbery particles and the concentration of rubber particles dispersed within the HIPS resin are believed to affect the impact strength of the HIPS resin.
The addition of the rubbery particles to PS to form HIPS tends to result in a reduction of the glossiness of the resin and the products made from the resin. The lack of gloss of conventional HIPS resins is often a disadvantage relative to materials such as acrylonitrile-butadiene-styrene (ABS) resin, as ABS generally has both high impact strength and high gloss. Many consumer products require a balance of both gloss and impact strength. Examples of such products include telephones, computers, and other consumer electronics.
Other requirements besides strength and gloss for these products is low cost and availability in large volumes. In particular, cost of the resin must be competitive with alternatives (such as ABS). Also, the resin must be available in large commercial volumes thus a large scale manufacturing process must be viable.
Gloss of conventional HIPS resins has been improved by using relatively small size rubber or elastomer particles, as opposed to large size particles. Further, it has been found that resins that have both small elastomer particles and large elastomer particles, in a bimodal size distribution, have both good gloss and high impact strength. See, for example, U.S. Pat. Nos. 4,282,334; 4,493,922 and 5,039,714. U.S. Pat. No. 5,294,656 also discloses a bimodal resin composition. The '656 resin comprises a styrene matrix containing small-sized particles having a core/shell structure with an average particle size of 0.1 to 0.4 microns, and large-sized particles having a cell structure with an average particle size of 0.8 to 2.0 microns.
U.S. Pat. No. 5,334,658 to Blumenstein et al. discloses a bimodal HIPS resin composition comprising 75 to 97% by weight polystyrene and 3 to 25% of a particulate elastomeric (co)polymer. 40 to 98% by weight of the particulate elastomeric (co)polymer in the form of capsule particles having a mean particle size of from 0.1 to 0.6 microns; and 1 to 60% of the remaining (co)polymer having a particle size of from 0.200 to 1.200 microns and having cell morphology; and from 40 to 99% by weight of the remaining (co)polymer having a mean particle size of from 1.2 to 8.0 microns also having cell morphology. The composition in Blumenstein et al. was made by mixing melts in an extruder.
U.S. Pat. No. 5,428,106 to Schrader et al. discloses a HIPS composition comprising 90 to about 55 wt. % polystyrene and 10 to about 45 wt. % grafted and occluded diene-based rubber particles. The rubber particles are composed of:
25 to about 80 weight percent having a capsule morphology and a volume average size of from 0.1 to 0.4 microns; and PA1 from about 75 to about 20 weight percent of rubber particles having an entanglement morphology and having a volume average particle size of from about 0.25 to 1 micron. PA1 25 to about 80 weight percent of rubber particles having a capsule morphology and a volume average particle size of from 0.1 to 0.4 microns; PA1 75 to about 20 weight percent entanglement particles; and PA1 1 to 25 weight percent of rubber particles having a cellular morphology and a volume average particle size of from about 0.6 to about 1.2 microns. PA1 a. Contacting a first styrene monomer feed and a styrene-butadiene copolymer feed in a first reaction zone under polymerization reaction conditions to form a first mixture; PA1 b. Controlling the reaction conditions in the first reaction zone so there is no phase inversion and no formation of capsule particles; PA1 c. Reacting the first mixture in a second reaction zone to form a substantial amount of the capsule particles and form a second mixture; and PA1 d. Contacting the second mixture in a third reaction zone with a polybutadiene under reaction conditions to form a substantial amount of the cellular particles. PA1 V.sub.Cell,T =V'.sub.Cell /V.sub.T, and V.sub.Cap,T =V'.sub.Cap /V.sub.T ; where PA1 R=V.sub.Cell,T /V.sub.Cap,T =V'.sub.Cell /V'.sub.Cap PA1 V.sub.Cell =V'.sub.Cell /(V'.sub.Cell +V'.sub.Cap)=R/(1+R) PA1 V.sub.Cap =V'.sub.Cap /(V'.sub.Cell +V'.sub.Cap)=1/(1+R)
The examples of this patent disclose making the claimed resin in a continuous, linear three stirred tube reactor system.
U.S. Pat. No. 5,491,195 also to Schrader et al. is a division of the same parent application as U.S. Patent 5,428,106 discussed above. '195 discloses a method of making the resin claimed in the '106 patent. '195 discloses the use of three stirred tube polymerization reactors connected in a series. '195 also discloses a composition comprising 90 to 55 weight percent polystyrene and 10 to about 45 weight percent diene-based rubber particles dispersed within the polystyrene matrix. The rubber particles are composed of:
Blumenstein et al. U.S. Pat. No. 5,334,658 does not disclose an in situ process to make bimodal HIPS.
Schrader et al. U.S. Pat. No. 5,428,106 discloses an in situ process to make a high gloss, high impact styrene resin but neither Schrader et al. U.S. Pat. No. 5,428,106 nor U.S. Pat. No. 5,491,195 disclose using such process to make a bimodal resin. The compositions of the resins disclosed in the Schrader et al. patents both comprise large amounts (75 to about 20 weight percent) of entanglement type particles. Schrader et al. '195 also discloses the presence of cellular type particles in the composition but the cellular particles are of a volume average particle size of from about 0.6 to about 1.2 microns.
U.S. Pat. No. 4,146,589 to Dupre discloses a method for making a bimodal HIPS in a mass polymerization process. Dupre forms a first partially polymerized solution containing rubber particles having an average diameter of about 0.5 to 1.0 microns in a first reaction zone. A second partially polymerized solution containing rubber particles having an average diameter of about 2 to 3 microns is formed in parallel in a second reaction zone. The first and second partially polymerized solutions are mixed together in a third reaction zone. The Dupre method requires two reactor trains in parallel where the rubber particles are formed prior to combining the two streams. The parallel reactor method of Dupre requires at least one additional reaction step than the process of the present invention.